1. Field of the Invention
The present invention relates to a recording apparatus in which recording is performed by ejecting ink to fly in the form of small droplets through ejection ports (orifices) and depositing the small droplets on the surface of a recording material, as well as a method of controlling the recording apparatus.
2. Description of the Related Art
Heretofore, as ink for ink jet recording, water-based ink has been primarily employed from the standpoints of, e.g., ensuring safety and eliminating a bad odor. There are known many types of ink prepared by dissolving or dispersing various water-soluble dyes or pigments in water or a mixture of water and a water-soluble organic solvent, and if necessary adding a moisture retaining agent, a dye dissolving aid, a fungicide, etc. Ink jet recording made using such ink has been rapidly developed in these years because of many advantages of, e.g., enabling high-speed recording to be easily realized as several thousands or more ink droplets can be ejected per second, generating less noise, ensuring easy production of a color image, providing a high resolution, and permitting an image to be printed on plain paper.
Further, with a recent trend toward the lower price, higher performance, and standardization of the GUI (Graphical User Interface) environment in the field of personal computers, there is increasing demand for better color development, higher quality, higher durability, higher resolution and higher speed in image recording using printers or the like. To meet such a demand, technical concepts have been proposed with an aim at holding down feathering, bleeding (color mixing) and other unfavorable properties by leaving color components as much as possible on the.paper surface and making the edges of recording dots sharper.
As the first example, Japanese Patent Laid-Open No. 58-13675 discloses a method of controlling absorption of ink and spread of recording dots into and over paper by addition of polyvinyl pyrrolidone into the ink. As the second example, Japanese Patent Laid-Open No. 3-172362 discloses a method of controlling absorption of ink and spread of recording dots into and over paper by addition of a specific micro-emulsion into the ink.
As the third example utilizing a sol-gel transition phenomenon of ink, Japanese Patent Laid-Open No. 62-181372, and No. 1-272623, as well as others disclose that ink can be prevented from permeating into paper by using an ink which is in the gel form at room temperature and transitions to the sol form upon heating, and recording an image with the ink applied in the sol state to a recording material, because the ink restores to the gel state upon cooling.
As the more recent fourth example, Japanese Patent Laid-Open No. 6-49399 discloses an ink which is added with a compound having a characteristic of thermally gelling in a reversible manner, thereby realizing good color development and fixing property, showing less feathering, and being superior in preservation and reliability of prints, as well as an ink jet recording method and apparatus both using the ink. The technical background of this related art is based on a phenomenon that as a solution of a particular water-soluble high molecule is gradually heated, water solubility of the high molecule is reduced and the solution becomes cloudy at a specific temperature (that is called a clouding point). Typical examples of such a high molecule include, for example, N-isopropyl acrylic amide, polyvinyl methyl ether, polyethylene oxide, and hydroxypropyl cellulose. Because these high molecules show solubility having a negative temperature coefficient, they are separated and precipitated from the solution at temperatures not less than the clouding point. In the precipitating state, viscosity of the solution is lowered due to generation of hydrophobic microgel. After being recorded on a recording material in the precipitating state, the solution restores its original viscosity with a temperature effect developed on the surface of the recording material. Thus, the increased viscosity holds down the ink from permeating into paper.
Meanwhile, as the fifth example, M. Croucher et al. point out the problems of conventional homogeneous ink and propose, as inkjet ink in future, an inhomogeneous ink utilizing latex (see M. D. Croucher and M. L. Hair; Ind. Eng. Chem. Res. 1989, 28, 1712-1718, "Design Criteria and Future Direction in Inkjet Ink Technology"). In addition, U.S. Pat. No. 4,246,154 discloses an ink wherein fine particles of a vinyl polymer colored with dyes are stabilized in the anionic form. U.S. Pat. No. 4,680,332 discloses an inhomogeneous ink containing an oil-soluble dye wherein a water-soluble polymer coupled to a non-ionic stabilizer is dispersed a liquid medium. Also, U.S. Pat. No. 5,100,471 proposes a water-based ink consisted of a solvent and colored particles each made up of a polymer core and a silica shell covalently bonded to a dye. This proposed ink has features of enabling colors to develop more sharply on paper, being stable against temperature, and providing high water-resistance.
Further, as the sixth example, Japanese Patent Laid-Open No. 3-240586 proposes a non-water-based ink wherein colored particles covered by a resin swelling with a dispersion medium, such as kerosene, are dispersed in the dispersion medium. It is suggested that this proposed ink is effective particularly in preventing feathering of ink images and clogging of nozzles for ejecting liquid droplets.
The above-stated first and second examples have a problem in fixing property because the ink is prevented from permeating into paper and is to left stand on the paper for a long time without undergoing permeation. Another problem is that there occurs mixing between different colors (i.e., bleeding).
The sol-gel transition ink shown as the third example has a problem that a recorded image may suffer from bleeding and transfer fouling because changes in preservation temperature of prints cause the ink to have fluidity and to flow out.
The ink added with a compound having a characteristic of thermally gelling in a reversible manner, shown as the fourth example, is not suitable for a method of recording an image at such a high speed of not more than 10 msec per pixel as required in ink jet recording, because a rise in viscosity with a temperature drop is too slow as a result of employing water-soluble cellulose ethers. Also, when used in ink jet recording, ink is required to have an upper limit of viscosity not more than 20 mPa.multidot.s at the time of ink ejection. The ink must be therefore employed with a low concentration enough to satisfy the above requirement, which makes it hard for the ink to produce the effect of increasing viscosity sufficiently.
On the other hand, of the fifth example group, the ink containing vinyl polymer fine particles stabilized in the anionic form has a problem that a pH range in which the ink can disperse stably is narrow and a selectable range of dyes is small consequently. Another drawback is that spread of recording dots on paper is too small to provide a satisfactory value of optical density (O.D.). Further, the ink is less effective in shortening a fixing time, though this effect is essential for high-speed recording, because a fixing mechanism of the ink depends on only evaporation and permeation as with conventional image forming means.
Another ink of the fifth example group, which contains an oil-soluble dye and in which a water-soluble polymer coupled to a non-ionic stabilizer is dispersed a liquid medium, has an enlarged selectable range of dyes, but is also less effective in shortening a fixing time because a fixing mechanism of the ink depends on only evaporation and permeation as with the above ink. In addition, this ink is disadvantageous in preventing mixing between different colors (i.e., bleeding) because it takes time until adjacent dots are fixed into a stable state.
Still another disperse ink having the polymer-core/silica-shell structure is superior in stability of pigment dispersion, but cannot provide a satisfactory value of O.D. because the ink has no special means for causing a color material to aggregate on the paper surface. Further, this ink is also less effective in shortening a fixing time because its fixing mechanism depends on only evaporation and permeation, thus accompanying a problem of bleeding.
The problem common to the above three types of ink of the fifth example group is that a recorded image has a poor abrasion property as a result of taking no account about adhesion of color material particles onto the paper surface.
The ink of the sixth example is problematic in points of safety, a bad odor and so on because kerosene is used as the dispersion medium.
Physical properties required for water-based ink, in particular, required for ejecting the ink in the form of small droplets for ink jet recording will be now explained below. The physical properties required for inkjet ink to be ejected in the form of small droplets are given by;
surface tension; &gt;20 dyne/cm (relating to a refill speed), PA1 viscosity; 1-20 mpa.multidot.s, PA1 pH: 3-10, and PA1 fixing time&lt;20 sec (preferably as short as possible).
Here consider transition of ink onto paper. Generally, there is known the Lucas-Washburn's equation about a transition phenomenon of a liquid onto paper. Assuming that the amount of the liquid transited is V, the index of paper roughness is Vr, the absorption coefficient is Ka, the transition time is T, and the wetting start time is Tw, the Lucas-Washburn's equation is expressed by the following formula (1) when the liquid is water:
V=Vr+KaT-Tw (1)
In the formula (1), Ka relates to physical properties of both paper and ink, and is expressed by the following formula (2): ##EQU1##
In the formula (2), r is the diameter of a capillary, .gamma. is the surface tension of the liquid, .theta. is the contact angle, and .eta. is the viscosity of the liquid.
As is apparent from the formula (1), to leave a color material on the paper surface, it is required to slow down permeation of a liquid, i.e., to reduce the absorption coefficient Ka (the evaporation time can be prolonged by reducing Ka). It is also apparent that, to the above end, physical properties of ink preferably have a smaller surface tension, a larger viscosity and a larger contact angle. But since there are restrictions in those physical properties of ink for ink jet recording, adjustment of Ka is not easy.
On the other hand, where the liquid is a non-aqueous solvent, e.g., ethanol, the wetting time Tw in the formula (1) can be ignored and therefore the fixing time can be shortened. However, since the absorption coefficient Ka is increased to promote an effect of permeating ink, a printed image is more susceptible to feathering. Further, since the term of cos.theta. in the formula (2) is determined depending on a combination of ink and paper, image quality depends on the types of paper. Thus, the ink using a non-aqueous solvent cannot satisfy a paper selection property.
The above-mentioned problems are believed to possibly occur also in conventional color-material dispersed ink so long as image formation depends on only evaporation and permeation.
With a view of solving the problems mentioned above, the inventors previously proposed it to use an ink for printing which contains a high molecule having viscosity that increases in a thermally reversible manner (Japanese Patent Laid-Open No. 8-333535).
Taking into account the above-stated restrictions being attributable to the fact that ink is a liquid in the homogenous state of a color material and a solvent regardless of temperature, the inventors proposed in the above Japanese Patent Application an ink which causes a state change triggered depending on temperature so that the color material and the solvent separately behave on a recording material.
To explain the state change in more detail, high molecule particles are isolatedly dissolved in the ink state at room temperature, but they aggregate at temperature higher than a certain value into a dense liquid having a high viscosity, and form a state where the color material is coupled to the high molecule. By applying the ink in the latter state to the recording material, recording is made with a dense color material phase left on the surface of the recording material, while a thin solvent phase permeates into the recording material. Also, the state change must be reversible to be adapted for a wide range of environment temperature under which recording is potentially made.
In practice, when small droplets of ink are ejected from a recording head, it is advantageous for high-speed recording that the ink has a low viscosity. A phenomenon of the above state change can be therefore realized by ejecting the ink in the state having a low viscosity in operation of the recording head, and recording the ink on a recording material heated up to above the transition temperature. Owing to the relationship of temperature of the ink droplets&lt;temperature of the recording material in the above case, at the moment the ink droplets adhere to the surface of the recording material, the surface of the recording material is cooled to provide a slight time lag in rising of the ink droplet temperature up to the transition temperature. During the lag time until reaching the transition temperature and showing a high viscosity, the ink droplets have a low viscosity and therefore the ink permeates into the recording material in accordance with the Lucas-Washburn's equation. This mechanism also serves as means for solving a problem that if the color material is all left on the surface of the recording material, a recorded image would have a poor abrasion property.
The transition temperature is preferably set to be higher than the environment temperature (room temperature) under which recording apparatus is usually employed, and to fall in the range of 35.degree. C. to 100.degree. C. to enhance the effect of increasing viscosity depending on temperature (i.e., to enlarge a temperature difference between before and after the state change). The transition temperature equal to 100.degree. C. or higher is not preferable because it would cause a notable increase in viscosity due to evaporation of water in the ink. As shown in a viscosity characteristic graph of FIG. 1, by way of example, the ink preferably has the transition temperature in the range of 46.degree. C. to 48.degree. C.
Meanwhile, as a method of ejecting ink for recording, there is known an ink jet type applying thermal energy to the ink and causing ink droplets to fly out through orifices. In such an ink jet method, a bubble is generated in the ink with the applied thermal energy, whereupon the ink droplets are given kinetic energy enough to eject through the orifices. When ejecting the above-proposed ink by the ink jet method, heating the average temperature of the ejected ink droplets up to the transition temperature is more efficient as a heating method than heating paper separately. However, when such an ink as requiring much heat to be applied for ejection, like the above-proposed ink, is ejected, the conventional heating method may not impart sufficient thermal energy to the ink. In some cases, before sufficient thermal energy Is transmitted to the ink, a bubble is abruptly generated and the ink is thermally isolated from a heater.
To solve the above problem, therefore, the inventors proposed a driving method for averaging heat flux transmitted from the heater surface to the ink in a controlled manner. As one example of such a driving method, the inventors showed a method of heating and ejecting the ink by using a train of five pulses. In that example, the driving voltage was set to 7.5 V (this voltage value itself has no direct meaning in relation to the actual process and is merely one instance because the amount of heat actually generated varies in the resistance value of a heater; that is, what has actual meaning is a value of heat flux explained below). As shown in FIG. 27, the ink was heated by a train of five pulses each having a crest value of 7.5 V.
In the above example, as shown in FIG. 28, heat flux was produced in the form of a pulse train corresponding to the driving voltage. A peak value of heat flux produced upon application of the driving voltage exceeded 100 [MW/m.sup.2 ] and, particularly, it rose to 140 [MW/m.sup.2 ] at the time a bubble was generated. This enabled the bubble to stably generate in volume sufficient to provide a strong ejection force even with the above-proposed ink. Also, as shown in FIG. 29, the ink being 10 .mu.m over the heater surface was heated up to 69.6.degree. C. until generation of a bubble. Thus, an effect of sufficiently heating the ink to be ejected was achieved.
The driving method of generating one bubble with a plurality of driving signals as stated above, however, is inconvenient in the case of ejecting ink through a number of nozzles. Although a printing head usually has a number of nozzles for realizing high-speed printing, a load imposed on a power supply is remarkably increased if heaters for those nozzles are energized at the same time. In the technical field of ink jet recording and thermal transfer recording, therefore, a block driving method has been widely used for the purposes of reducing the load imposed on the power supply and realizing high-speed printing. With the block driving method, the nozzles are divided into a plurality of blocks which are energized with a small time lag therebetween, and arrangement of the nozzles is shifted to eject ink in proper sequence so that the printing result is not affected.
Where a pulse for driving one nozzle is given by a single pulse, or where, though it consists of a plurality of pulses, these pulses have so short time intervals as to be virtually regarded as a single pulse, the above block driving method does not accompany a notable difficulty. For example, when 64 nozzles are divided into 8 blocks each comprising 8 nozzles and each nozzle is driven by a pulse of 3 .mu.s, respective first nozzles of the 8 blocks are driven at the same time, and after several .mu.s, respective second nozzles of the 8 blocks are driven at the same time. Subsequently, respective third to eighth nozzles of the 8 blocks are all driven in a like manner. This makes it possible to drive all the nozzles in a shatter time, alleviate a shift in printing, and reduce the load imposed on the power supply upon simultaneous driving.
Where a driving pulse applied to one nozzle is given by a train of five pulses and the time intervals between the five pulses are predetermined like the above-explained example, however, it is difficult to freely apply the block driving method. As one example for applying the driving signals, which is not the known art, but taken into account in studies made for accomplishing the present invention, FIG. 30 shows a pulse train consisted of three pre-pulses (for preheating) each having a width of 1 .mu.s and a driving pulse proper (pulse for generating a bubble directly) having a width of 3 .mu.s, with intervals between the pulses being each 7 .mu.s. The illustrated example is to drive a nozzle array comprising 8 blocks. In the block driving method, it is preferable from the standpoint of causing no shifts in a recorded image that driving of all the blocks is completed as quick as possible. To this end, therefore, the block driving method is required to be executed such that the first signal of the second block, for example, locates between the first and second signal of the first block. In the example of FIG. 30, the driving signals of the second block are issued with a delay of 1 .mu.s from the driving signals of the first block, the driving signals of the third block are issued with a delay of 1 .mu.s from the driving signals of the second block, and so on. The driving signals of the eighth block are finally issued with a delay of 1 .mu.s from the driving signals of the seventh block.
In the illustrated example, because the driving signals of the respective blocks are shifted 1 .mu.s from one another, the preheating pulses each having a width of 1 .mu.s are not overlapped. But the driving pulse has a width of 3 .mu.s, and therefore the driving pulse of the first block overlaps with the driving pulse of the second block for a period of 2 .mu.s. During this period, a current value is doubled and a power supply requires a current capacity twice as much. In the illustrated example, because the driving pulse of the third block further overlaps with the driving pulses of the first and second blocks, the power supply actually requires a current capacity triple as much. If the respective blocks are driven with a time lag of 3 .mu.m therebetween to prevent the driving pulses from overlapping with each other, overlaps between the driving pulses are avoided, but the driving pulse of the first block overlaps with the third preheating pulse of the fourth block and subsequently the second preheating pulse of the seventh block. Accordingly, the power supply requires a current capacity twice as much for a period of 2 .mu.s.